A simple electrochemical route to metallic phase trilayer MoS2: evaluation as electrocatalysts and supercapacitors

Abstract

The development of a simple, scalable and reproducible technique for the synthesis of two-dimensional MoS₂ nanosheets is of paramount importance in the field of catalysis and energy storage devices. Current routes to produce MoS₂ nanosheets in reasonable quantities involve either solution exfoliation of bulk MoS₂ or intercalation of organo-lithium into bulk MoS₂, which is then subsequently exfoliated by immersing it in water. The former process produces semiconducting 2H-MoS₂ nanoplatelets with smaller lateral flake sizes whereas the latter process produces highly conducting metallic (1T) phase monolayer MoS₂. 1T-MoS₂ nanosheets have high catalytic activity for the hydrogen evolution reaction (HER) and are efficient electrode materials for supercapacitors when compared to the 2H phase. However, the feasibility of producing 1T-MoS₂ by organolithium intercalation is undermined by the long reaction time (2–3 days) and by its pyrophoric nature. We report a simple, bench-top electrochemical process to produce exfoliated metallic phase MoS₂ within two hours. By using an inert Li salt (LiClO₄) as a source of lithium and a Pt counter electrode, an electrochemically lithium intercalated MoS₂ electrode was obtained, which was subsequently exfoliated by immersing it in water. Characterization of the exfoliated product using a variety of methods confirmed the formation of the 1T phase. Remarkably, flake thickness measurement using atomic force microscopy and high-resolution transmission electron microscopy revealed that the majority of the nanosheets are trilayers. The 1T-MoS₂ nanosheets showed enhanced electrocatalytic activity towards hydrogen evolution compared to 2H-MoS₂ and are efficient materials for supercapacitor applications. Coin cells constructed from a 1T-MoS₂–graphene composite achieved a volumetric capacitance of over 560 F cm⁻³ in an aqueous system and 250 F cm⁻³ in a non-aqueous electrolyte with capacity retention of over 90% after 5000 cycles. This process is readily scalable and should ultimately support the production of metallic MoS₂ for various applications. It can also be extended to produce 2H-MoS₂ nanosheets by heating the exfoliated 1T phase.